Really ??, because this setting is very unlikely to alter/need adjustment just like that and if you change it it'll change your colour balance (resulting in your white not being white anymore and/or black not being black anymore).
If it is a bad tube changing the setting may ''fix'' it for the time being...
@Nobby and ts and others: just a few hours ago i had a BV3 with the same effect and indeed one of the sick tubes.
However, inspired by another forum i'v experimented a bit and found a method which seems to work most of the time, up 'till now i fixed the tubes of a AV5, 2x Avant 28 curved, 2x RF 32 and now BV3-32.
It takes a bit of care and explaining but it is fairly simple, the trick is actually making the heather glow very strong and tapping the tube.
I have no idea how long this ''repair will hold but the AV5 is already in use for weeks without a priblem, so are the 2 28 Avants...
So Wink, i also suspect your tube but if it appears to be that it's not a 100% lost case.